The objective of the proposed research activity is to analyze if the soft and microbial plaque is the major etiologic factor in progressive destructive periodontitis. Twenty Beagle dogs are used; 10 experimental and 10 control dogs. During a 4 year period the animals have been on a soft diet program which allows the formation of abundant amounts of soft and hard tooth deposits. The teeth of the control dogs have twice daily been subjected to meticulous brushing while the teeth of the experimental group dogs have not been cleaned. The clinical findings show that it is possible in dogs to induce an inflammation in the gingiva, which gradually propagates into and destroys the supporting tissues, simply by allowing plaque to form and calcify on the gingival part of tooth surfaces. In control dogs with clean teeth no signs of gingivitis or periodontal breakdown have been discovered. Gingival tissues obtained after 2, 6, 12, 24, 36 and 48 months of plaque accumulation in experimental group dogs contain a connective tissue portion which is infiltrated by leukocytes (ICT). In sections representing 24 to 48 months of gingivitis this ICT portion has its apical border located at or apical to the CEJ. In the 48 month sections the mean distance (assessed in the microscope) between (1) the cemento-enamel junction (CEJ) and the apical cells of the junctional epithelium was 2674,um (control 13,um) (2) the CEJ and the alveolar bone crest was 3.31 mm (control 1.02 mm).